Unsaturated polyesters are industrially important materials which have found broad application for the manufacture of automotive parts, boats, appliance parts, furniture, and other such diverse products. Unsaturated polyesters are low molecular weight polymers prepared by the melt phase reaction of a glycol such as ethylene glycol, propylene glycol, neopentyl glycol, and the like; and an unsaturated dicarboxylic acid such as fumaric acid or maleic anhydride. Additionally, other dicarboxylic acids may be used to effect desired properties. For the preparation of the polyester, the glycol, or a blend of glycols, is employed in slight excess of the molar equivalency of the sum of the acid components. The excess glycol is required because of loss of glycol by adventitious decomposition reactions as well as some loss by distillation. When the polyester achieves the appropriate degree of polymerization, the heat is removed and the resin is dissolved in styrene monomer. Styrene serves both as a solvent vehicle for the resin as well as the crosslinking agent. The resin is generally combined with desired amounts of peroxide initiator, fillers, fiberglass reinforcement, and other materials necessary to prepare molding compounds that can be subsequently formed and crosslinked into useful products. Fiberglass reinforced unsaturated polyester compositions generally exhibit a desirable balance of physical properties. However, in the presence of organic solvents, the crosslinked unsaturated polyesters are highly susceptible to vigorous solvent sorption and swelling. The swelling stresses which develop can quickly lead to resin fracture and a severe loss of mechanical properties. For this reason, conventional unsaturated polyester resins are not recommended for the fabrication of pipes and tanks for contact and handling of polar organic solvents. This invention, however, describes a novel composition of matter that can be prepared and processed by conventional methods, but unexpectedly exhibits high resistance to attack by organic solvents.